The paste hydration of brownmillerite with and without gypsum: a time resolved synchrotron diffraction study at 30, 70, 100 and 150 °C

Abstract

Synchrotron energy dispersive diffraction has been used to monitor the mineral transformations which occur in the paste hydration of brownmillerite Ca2AlFeO5 in a closed hydrothermal environment at temperatures in the range 30 to 150 °C. In the absence of sulfate brownmillerite reacts rapidly to form a metastable hexagonal hydrate with a basal spacing of 1.07 nm. As temperature increases the metastable hydrate transforms to the stable hydrogarnet phase Ca3(Al,Fe)2(OH)12 with an Al/(Al + Fe) ratio of about 0.4. The lack of iron in the metastable hydrate suggests a through-solution mechanism while the iron content of the hydrogarnet indicates it must form on the surface of the brownmillerite, probably in contact with the Fe-rich residue remaining after dissolution. The addition of gypsum CaSO4·2H2O increases the rate of brownmillerite hydration. Ettringite Ca6Al2(SO4)3(OH)12·26 H2O is formed at ambient temperature and is replaced by calcium aluminium monosulfate-14 hydrate Ca4Al2O6(SO4)·14H2O at elevated temperatures (70, 100 and 150 °C). Transient increases in gypsum accompany this transformation and all three phases can co-exist at 70 °C. Monosulfate-14 is stable at 150 °C. Increases in brownmillerite peak intensities accompany the conversion of ettringite to monosulfate and are a result of sedimentation. This has implications for paste rheology and slurry design in oilwell cements at elevated temperatures.